Radar level gauges are in wide use for measuring the filling level of a product contained in a tank. Radar level gauging is generally performed by propagating an electromagnetic transmit signal towards the product contained in the tank, and receiving an electromagnetic surface reflection signal resulting from reflection of the transmit signal at the surface of the product.
The transmitted electromagnetic signal may be radiated towards the product contained in the tank, or may be guided towards and into the product by a transmission line probe. The latter is often referred to as Guided Wave Radar (GWR).
Based on the transmit signal and the surface reflection signal, the distance to the surface of the product can be determined. The filling level can be deduced from this distance.
Radar level gauges are often classified as either pulsed systems or FMCW-systems. In pulsed systems, pulses are transmitted towards the surface of the product, and the distance to the surface is determined based on the time-of-flight of the pulse to the surface and back to the radar level gauge. In FMCW-systems, a signal with a time-varying frequency is transmitted towards the surface and the distance is determined based on the frequency (and/or phase) difference between a transmitted signal and a simultaneously received signal. So-called pulsed FMCW-systems also exist.
Radar level gauges are in some cases used for applications where malfunction of the radar level gauge could result in dangerous situations.
For example, a radar level gauge with overfill prevention functionality must be extremely reliable.
Various measures are taken to ensure the reliability of radar level gauges, in particular radar level gauges with overfill prevention functionality, and to thereby reduce the risk of dangerous situations, such as overfill. For instance, radar level gauges and other important devices may be designed to fulfill a certain Safety Integrity Level (SIL) as defined by the international standard IEC/EN 61508. According to this standard, safety related systems may fulfill the requirements for different Safety Integrity Levels from SIL1 to SIL4, where SIL4 represents the highest safety integrity level and SIL1 represents the lowest safety integrity level.
The SIL-rating of a system is related to the probability of failure on demand, which is in turn a function of the failure rate and the time between proof tests. To maintain a certain SIL-rating, it is thus necessary to perform proof tests at a regular interval specified in accordance with the SIL-rating. For example, proof tests may need to be performed annually.
Proof tests are generally specified by the manufacturer of the SIL-rated system and it is the responsibility of the user of the system that the proof tests are carried out properly.